The invention related to methods of joining thermoplastic pipes to couplings.
An example of such a method is disclosed in British patent specification number GB-A-1596112. That specification describes a pipe coupling which comprises an insert having a tubular body provided with a plurality of circumferential grooves in its outer surface, each groove having in axial section substantially straight sides and a straight base, the angle between the base and each side being obtuse, a pipe end into which the insert has been inserted, and a sleeve engaged around the pipe end such that the pipe end is compressed between the insert and the sleeve, characterised in that the diameter of the sleeve is not adjustable, the external diameter of the insert is greater than the internal diameter of the pipe in its free state, and the internal diameter of the sleeve is at least as great as the external diameter of the pipe in its free state.
The method of joining the thermoplastic pipe to the coupling comprises pushing the sleeve onto the pipe and away from the pipe end, the internal diameter of the sleeve being at least as great as the external diameter of the pipe, an insert is forced into the end of the pipe, the internal diameter of the pipe in its free state being less than the external diameter of the insert, whereby the external diameter of the pipe end around the insert is increased to be greater than the internal diameter of the sleeve, and the sleeve is forced to the end of the pipe so that the pipe end is tightly gripped between the insert and the sleeve.
In that method, the internal diameter of the sleeve is at least as great as the external diameter of the pipe in its free state.
Another example of such a method is disclosed in a specification published by the former German Democratic Republic numbered GDR-A-128216. The specification indicates that it is possible to taper the insert.
The invention is particularly, though not exclusively, concerned with joining reinforced pipes to couplings. The reinforcement prevents such pipes being expanded or only being capable of very small expansions. In the methods described in GB-A-1596112 and GDR-A-128216 the external diameter of the insert is sufficiently greater than the internal diameter of the pipe in its free state as to require substantial expansion of the pipe when the insert is pushed into the end of the pipe. On the other hand, the sleeve, which is pushed over the pipe end beforehand, is an easy fit on the external diameter of the pipe in its free state.
When the thermoplastic pipe is joined to the coupling the pipe is compressed between the sleeve having an internal diameter equal to or greater than the external diameter of the pipe and the insert which has an external diameter greater than the internal diameter of the pipe.
According to the invention, a method of joining a thermoplastic pipe to a coupling comprising a tubular insert having external circumferential projections separated by circumferential grooves and an external shoulder and a tubular sleeve having a largest internal diameter less than the external diameter of said pipe, comprises causing relative approach of said sleeve and said pipe said pipe being constricted as said sleeve passes over it and then subsequently recovering to establish an end portion of said pipe which is unconstricted by said sleeve, inserting said insert into said end portion, and causing relative movement between said sleeve and said end until said end portion is gripped between said projections and said sleeve.